Method for making and using a drilling fluid

ABSTRACT

A drilling fluid and method for drilling in a coal containing formation. The method includes: providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt; circulating the drilling fluid through the well; and drilling into a coal seam.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/776,914 filed Feb. 26, 2013 which is presently pending. U.S. Ser. No.13/776,914 is a divisional of U.S. application Ser. No. 12/529,583 filedDec. 22, 2009, now U.S. Pat. No. 8,387,723 issued Mar. 5, 2013, which isa 371 of PCT/CA2008/00425 filed Mar. 3, 2008, which claims benefit ofU.S. 60/892,672 filed Mar. 2, 2007.

FIELD

This invention relates to methods and fluids used for drilling andcompleting oil wells.

BACKGROUND

The process of drilling a hole in the ground for the extraction of anatural resource requires a fluid for removing the cuttings from thewellbore, lubricating and cooling the drill bit, controlling formationpressures and maintaining hole stability.

Many earth formations contain coal seams through which a wellbore mustbe drilled to either access the coal itself or reservoirs of interestbelow the coal.

For coal bed methane (CBM) wells, minimization of formation damage isparamount given the lower permeability of coal seams than conventionalreservoirs. A fluid that minimizes the formations damage and reduceswhole mud loss by limiting the invasion into the cleats and fracturesand permits easy flow back has been developed, termed herein the mixedmetal-viscosified drilling fluids including mixed metal oxide (MMO),mixed metal hydroxide (MMH) and combinations of mixed metal oxide andhydroxide (MMOH). The mixed metal-viscosified drilling fluids contain amixed metal viscosifier, which is an inorganic particle based onmagnesium/aluminum oxides and/or hydroxides. The mixed metal particleshave a cationic character and react electrostatically with clayparticles. Mixed metal-viscosified drilling fluids include anaqueous-based mixture of at least one of the mixed metal moieties and anamount of bentonite. The rheology of mixed metal-viscosified drillingfluids limits fluid invasion into the formation due to high viscositybut the main formation protection comes from the formation of anexternal filter cake that is easy to remove. Simple displacement towater or brine should be sufficient for the well to flow back and removethe filter cake.

Unfortunately, however, the rheology of mixed metal-viscosified drillingfluids has broken down when coming into contact with coal finesgenerated from drilling into coal seams, especially young coal. When thedrilling fluid comes in contact with coal fines generated by drillingthrough the seams, the fluid thins, moving toward the rheology of waterand therefore loses many of its beneficial properties. Since coal seamsare, in fact, often considered loss zone formations, and are weak andfriable, the unsuitability of mixed metal-viscosified drilling fluidsfor drilling in coal containing formations is particularly problematic.

SUMMARY OF THE INVENTION

In accordance with a broad aspect of the present invention, there isprovided a method for drilling in a coal containing formation, themethod comprising: providing a mixed metal-viscosified drilling fluidincluding at least 1% potassium salt; circulating the drilling fluidthrough the well; and drilling into the coal seam.

In accordance with another broad aspect of the present invention, thereis provided a drilling fluid comprising: an aqueous mixture of bentoniteand a mixed metal viscosifier with a pH above about pH 10; and at least1% potassium salt.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of example. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the detailed description and examples are to be regarded asillustrative in nature and not as restrictive.

DESCRIPTION OF VARIOUS EMBODIMENTS

The detailed description and examples set forth below are intended as adescription of various embodiments of the present invention and are notintended to represent the only embodiments contemplated by the inventor.The detailed description includes specific details for the purpose ofproviding a comprehensive understanding of the present invention.However, it will be apparent to those skilled in the art that thepresent invention may be practiced without these specific details.

Until now mixed metal (MMO, MMH and MMOH) viscosified drilling fluidshave been used generally unsuccessfully in coal seams due to the fluidthinning effect from the coal. It is believed that the polyanionicnature of coal fines, such as of lignite and lignosulfonates, interferewith the electrostatic interactions of the mixed metal moiety and thebentonite in the drilling fluid, sometimes resulting in a completecollapse of the fluid's rheology.

We have determined that some salts reduce or prevent the thinning effectfrom drilling coals with MMO, MMH and MMOH viscosified fluids. Potassiumsalts including one or more of potassium sulfate, potassium chloride,potassium acetate and potassium formate may substantially maintain therheology of mixed metal-viscosified drilling fluids when drilling withcoal contaminants. Such salts may add a benefit of shale swellinginhibition, possibly as a result of the presence of the potassium ionfrom the salt.

A wide range of potassium salt concentrations, such as concentrationsgreater than 1% (weight by volume), may be effective in the mixedmetal-viscosified drilling fluid. Generally concentrations of 1-10%(weight by volume) salt and, for example, 1-5% salt (weight by volume)concentrations have been found to be both effective for stabilizing thedrilling fluid against adverse rheological changes due to coalcontamination and advantageous in terms of economics. The amount of saltadded to the drilling fluid may be determined by the amount of coal tobe drilled and/or by the shale reactivity. For example, younger coals,more so than older coals, tend to create greater rheological instabilityfor mixed metal-viscosified drilling fluids and, thus, higherconcentrations (for example greater than 3% and for example 3-10%) ofpotassium salts in the drilling fluid may be useful. Also, if it isdetermined that there are significant coal deposits through which thewell must be drilled, again higher concentrations of potassium salts maybe useful.

Although the salt may be added after the coal contamination occurs, itis recommended to pre-treat the system for best results. In oneembodiment, for example, the surface hole can be drilled down toapproximately the level of the first coal deposit using any drillingfluid of interest, including for example, prior art mixedmetal-viscosified drilling fluids. When it is determined that the coalseam is close below bottom hole or when the coal seam has been reached,the drilling fluid may be changed over to a drilling fluid according tothe present invention, including a mixed metal-viscosified drillingfluid containing an amount of a potassium salt.

Alternately, the borehole may be drilled down to and through a coal seamusing a drilling fluid according to the present invention. For example,the entire well substantially from surface, which it will be appreciatedmay include drilling from surface or from below the overburden or afterthe casing point, may be drilled using a drilling fluid according to thepresent invention.

After drilling through the coal seams in the path of the borehole, thepresent drilling fluid may continue to be used for the remainder of thewellbore or other drilling fluids may be used. However, if coal finesmay continue to become entrained in the drilling fluid, for examplewhere a coal seam remains open to contact by the drilling fluid, it maybe useful to continue using the present drilling fluid until drilling iscomplete or the possibility of coal contamination is eliminated. Ifdesired, the drilling fluid returning to the mud tanks at surface may bemonitored to determine the concentration of potassium salt therein, aswell as other parameters, to ensure that appropriate levels and fluidcharacteristics are maintained. For example, any one or more of thebentonite, mixed metal viscosifier, base, or potassium salt may be addedduring drilling to adjust the drilling fluid parameters. In oneembodiment, for example, an amount of mixed metal viscosifier may beadded to the fluid during the course of a drilling operation wherereactive formations are drilled and drill cuttings become incorporatedto and change the rheology of the drilling fluid. In such a case, theaddition of an amount of mixed metal viscosifier can cause the viscosityof the fluid to increase.

As will be appreciated, the drilling fluid may be circulated through thedrill string, drill bit and well bore annulus while drilling.Circulation of the drilling fluid may continue even when drilling isstopped in order to condition the well, prevent string sticking, etc.

During the drilling and circulation, the yield point of the drillingfluid may be maintained above 10 Pa to provide advantageous effects.

Mixed metal-viscosified drilling fluids include bentonite and a mixedmetal viscosifier in water and are pH controlled.

Bentonite is commonly used in drilling fluids and its use will be wellunderstood by those skilled in the art. While various forms of bentonitemay be used, bentonites that contain polyanionic additives or impuritiesshould be avoided, with consideration as to the electrostaticinteraction of the bentonite and MMOH. An untreated bentonite may beparticularly useful. Such a bentonite may be known commercially asuntreated bentonite with a high content of sodium montmorillonite oruntreated Wyoming bentonite.

Mixed metal viscosifiers are commercially available such as from BASFOilfield Polymers Inc. under the trademark Polyvis™.

Generally, mixed metal-viscosified drilling fluids may include lowconcentrations of bentonite (for example, about 15 to 45 kg/m3 or 20 to40 kg/m3 bentonite in water). Considering that many bentonite based(non-mixed metal) drilling fluids can contain many multiples more (i.e.two to four times) bentonite than in a mixed metal-viscosified drillingfluid, it can be appreciated that the viscosity generated using such lowconcentrations of bentonite for mixed metal-viscosified drilling fluidsmight be insufficient for hole cleaning. The addition of mixed metaloxide, mixed metal hydroxide or mixed metal oxide and hydroxide at aweight ratio of 1:8 to 1:12 or 1:9.5 to 1:10.5 to the bentonite producesa stable fluid when the pH is initially maintained above about 10.0 andpossibly between about 10.5 and 13, as may be achieved by addition ofcaustic soda, caustic potash, potassium carbonate and/or soda ash. Oncethe bentonite/mixed metal viscosifier reaction is complete and a gel isformed, it appears that the pH can be lowered to pH 9 or possibly evenlower without any significant loss in viscosity.

In one embodiment, a mixed metal-viscosified drilling fluid may includean aqueous mixture of about 30 kg/m3 bentonite, a mixed metal moiety ina quantity of about 1:10 MMO, MMH or MMOH to bentonite, pH controlled togreater than pH 11 and 1 to 5% potassium salt.

Additives for fluid loss control, lost circulation, etc. may be added tothe drilling fluid mixture, as desired. Non or minor-ionic additives maybe most useful. Some examples may include starch for fluid lossreduction, organophillic lost circulation materials (LCM), etc. Simpletesting may verify the compatibility of any particular additive with thedrilling fluid.

To produce the drilling fluid, the bentonite may first be hydrated inwater. Then the mixed metal moiety is added and pH is adjusted. Thepotassium salt can be added to the aqueous mixture of bentonite andmixed metal any time before it is needed for drilling with coalcontamination. Additives such as LCM, fluid loss control agents, etc.can also be added when appropriate, as will be appreciated.

A typical drilling fluid formulation may be according to Table 1.

TABLE 1 A typical drilling fluid useful for drilling in coal-containingformations Product Concentration Notes Untreated bentonite 30 kg/m3Prehydrate first in fresh water MMH or MMO or MMOH 3 kg/m3 Caustic Soda0.5 to 1 kg/m3 To control pH at 11-12.5 Potassium Sulfate 20 to 50 kg/m3Starch 5 to 10 kg/m3

The following examples are included for the purposes of illustrationonly, and are not intended to limit the scope of the invention orclaims.

EXAMPLES Example I

In the following examples, drilling fluids were prepared according tothe sample descriptions by hydrating the bentonite, adding the mixedmetal moiety and adjusting the pH, as needed. Thereafter, any additives,including potassium salt if any, were added.

To simulate coal contamination, lignite was added.

The rheological properties have been tested using a Fann 35 andBrookfield viscometers.

TABLE 2 Composition of Sample #1 Products Sample #1 Untreated Bentonite30 kg/m3 MMH  3 kg/m3 Caustic 0.5 kg/m3  Starch 10 kg/m3

TABLE 3 Results without the addition of Salt Sample #1 + Sample #1 + 5kg/m3 15 kg/m3 Mud Properties Sample #1 Lignite Lignite 600 RPM 86 47 43300 RPM 64 29 25 200 RPM 53 21 18 100 RPM 40 13 10  6 RPM 19 2 1.5  3RPM 17 1 1  10 sec Gel (Pa) 8 1 0.5 PV (mPa * s) 22 18 18 YP (Pa) 21 5.59 LSRV (cP) 54,000 12,000 0 Temperature (° C.) 22.8 22.3 23.0

TABLE 4 Results using Potassium Chloride Sample #1 + Sample #1 + 2%KCl + 2% KCl + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 2% KClLignite Lignite 600 RPM 66 47 44 300 RPM 52 31 27 200 RPM 46 23 21 100RPM 38 16 14  6 RPM 18 4 3  3 RPM 16 3 2  10 sec Gel (Pa) 7 2 1.5 PV(mPa * s) 14 16 17 YP (Pa) 19 7.5 5 LSRV (cP) 25,000 12,000 9,000Temperature (° C.) 21.6 22.1 22.3

TABLE 5 Results using Potassium Acetate Sample #1 + Sample #1 + 2% Pot.Acetate + 2% Pot. Acetate + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties2% Pot. Acetate Lignite Lignite 600 RPM 66 52 48 300 RPM 47 38 35 200RPM 39 32 29 100 RPM 30 25 22  6 RPM 12 10 10  3 RPM 8 8 7  10 sec Gel(Pa) 4 4 4 PV (mPa * s) 13 14 13 YP (Pa) 20 12 11 LSRV (cP) 31,00020,000 12,000 Temperature 23.2 23.3 23.2 (° C.) Note: Lignite dissolvesslower.

TABLE 6 Results using Potassium Formate Sample #1 + Sample #1 + Sample#1 + 2% Pot. Formate + 2% Pot. Formate + 2% Pot. 5 kg/m3 15 kg/m3 MudProperties Formate Lignite Lignite 600 RPM 66 47 42 300 RPM 53 32 28 200RPM 47 26 22 100 RPM 38 18 16  6 RPM 19 6 5  3 RPM 18 4 4  10 sec Gel(Pa) 7 2 2 PV (mPa * s) 13 15 14 YP (Pa) 20 8.5 7 LSRV (cP) 21,00013,000 12,000 Temperature 22.1 22.3 22.6 (° C.)

TABLE 7 Results using Calcium Nitrate Sample #1 + 2% Sample #1 + Sample#1 + Calcium Nitrate + 2% Calcium Nitrate + 2% Calcium 5 kg/m3 15 kg/m3Mud Properties Nitrate Lignite Lignite 600 RPM 60 57 47 300 RPM 46 42 34200 RPM 38 34 28 100 RPM 31 27 22  6 RPM 12 11 7  3 RPM 9 9 5  10 secGel (Pa) 5 5 3 PV (mPa * s) 14 15 13 YP (Pa) 16 13.5 10.5 LSRV (cP)33,000 23,000 22,000 Temperature 21.5 22.1 22.7 (° C.) Note: Lignitedissolves slower.

TABLE 8 Results using Calcium Chloride Sample #1 + Sample #1 + 2%Calcium 2% Calcium Sample #1 + Chloride + Chloride + 2% Calcium 5 kg/m315 kg/m3 Mud Properties Chloride Lignite Lignite 600 RPM 61 51 47 300RPM 44 35 34 200 RPM 36 30 29 100 RPM 27 22 23  6 RPM 10 8 8  3 RPM 8 76  10 sec Gel (Pa) 3.5 3.5 3 PV (mPa * s) 17 16 13 YP (Pa) 13.5 9.5 10.5LSRV (cP) 27,000 23,000 22,000 Temperature (° C.) 24.4 24.4 24.2 Note:Lignite dissolves slower.

TABLE 9 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample#1 + 2% Pot. Sulfate + 2% Pot. Sulfate + 2% 5 kg/m3 15 kg/m3 MudProperties Pot. Sulfate Lignite Lignite 600 RPM 75 42 34 300 RPM 60 2921 200 RPM 52 24 16 100 RPM 41 18 11  6 RPM 21 8 2.5  3 RPM 19 7 2  10sec Gel (Pa) 9 4 2.5 PV (mPa * s) 15 13 13 YP (Pa) 22.5 8 4 LSRV (cP)32,000 30,000 25,000 Temperature (° C.) 24.4 24.0 21.3

TABLE 10 Results using Potassium Chloride Sample #1 + Sample #1 + 5%KCl + 5% KCl + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 5% KClLignite Lignite 600 RPM 61 52 46 300 RPM 49 39 35 200 RPM 45 35 32 100RPM 42 32 30  6 RPM 16 15 15  3 RPM 12 11 10  10 sec Gel (Pa) 6 6 5 PV(mPa * s) 12 13 11 YP (Pa) 18.5 13 12 LSRV (cP) 30,000 18,000 21,000Temperature (° C.) 20.1 20.1 20.1

TABLE 11 Results using Potassium Acetate Sample #1 + Sample #1 + 5% Pot.Acetate + 5% Pot. Acetate + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties5% Pot. Acetate Lignite Lignite 600 RPM 63 48 44 300 RPM 55 37 36 200RPM 51 36 34 100 RPM 47 34 32  6 RPM 14 20 16  3 RPM 9 11 11  10 sec Gel(Pa) 5 5 6 PV (mPa * s) 8 11 8 YP (Pa) 23.5 13 14 LSRV (cP) 27,00014,000 33,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignite dissolvesslower.

TABLE 12 Results using Potassium Formate Sample #1 + Sample #1 + Sample#1 + 5% Pot. Formate + 5% Pot. Formate + 5% 5 kg/m3 15 kg/m3 MudProperties Pot. Formate Lignite Lignite 600 RPM 50 46 42 300 RPM 40 3333 200 RPM 37 30 30 100 RPM 32 28 29  6 RPM 9 9 14  3 RPM 5 8 10  10 secGel (Pa) 3 4 5 PV (mPa * s) 10 13 9 YP (Pa) 15 10 12 LSRV (cP) 30,00029,000 31,000 Temperature 20.1 20.1 20.1 (° C.)

TABLE 13 Results using Calcium Nitrate Sample #1 + 5% Sample #1 + Sample#1 + Calcium Nitrate + 5% Calcium Nitrate + 5% Calcium 5 kg/m3 15 kg/m3Mud Properties Nitrate Lignite Lignite 600 RPM 58 49 44 300 RPM 52 42 38200 RPM 50 41 37 100 RPM 47 35 32  6 RPM 12 11 14  3 RPM 8 8 8  10 secGel (Pa) 5 4.5 4.5 PV (mPa * s) 6 7 6 YP (Pa) 23 17.5 16 LSRV (cP)35,000 43,000 23,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignitedissolves slower.

TABLE 14 Results using Calcium Chloride Sample #1 + Sample #1 + 5%Calcium 5% Calcium Sample #1 + Chloride + Chloride + 5% Calcium 5 kg/m315 kg/m3 Mud Properties Chloride Lignite Lignite 600 RPM 63 48 43 300RPM 50 37 34 200 RPM 42 34 31 100 RPM 35 29 29  6 RPM 13 12 13  3 RPM 109 11  10 sec Gel (Pa) 6.5 6.5 7 PV (mPa * s) 13 11 9 YP (Pa) 18.5 1311.5 LSRV (cP) 40,000 37,000 27,000 Temperature (° C.) 20.1 20.1 20.1Note: Lignite dissolves slower.

TABLE 15 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample#1 + 5% Pot. Sulfate + 5% Pot. Sulfate + 5% 5 kg/m3 15 kg/m3 MudProperties Pot. Sulfate Lignite Lignite 600 RPM 165 128 91 300 RPM 150115 76 200 RPM 143 109 71 100 RPM 131 100 63  6 RPM 85 67 42  3 RPM 3758 39  10 sec Gel (Pa) 16 29 22 PV (mPa * s) 15 13 15 YP (Pa) 77.5 5130.5 LSRV (cP) 100,000+ 80,000 67,000 Temperature (° C.) 20.1 20.1 20.1

TABLE 16 Results using Sodium Sulfate Sample #1 + 2% Sample #1 + Sample#1 + Sodium Sulfate + 2% Sodium Sulfate + 2% Sodium 5 kg/m3 15 kg/m3 MudProperties Sulfate Lignite Lignite 600 RPM 179 39 31 300 RPM 155 25 19200 RPM 143 20 15 100 RPM 123 14 9  6 RPM 72 8 3  3 RPM 63 7 2  10 secGel (Pa) 31 5 2.5 PV (mPa * s) 24 14 13 YP (Pa) 65.5 5.5 4 LSRV (cP)90,000 50,000 28,000 Temperature 22.0 22.0 22.0 (° C.)

TABLE 17 Results using Sodium Sulfate Sample #1 + 5% Sample #1 + Sample#1 + Sodium Sulfate + 5% Sodium Sulfate + 5% Sodium 5 kg/m3 15 kg/m3 MudProperties Sulfate Lignite Lignite 600 RPM 207 48 33 300 RPM 174 38 22200 RPM 152 35 18 100 RPM 124 31 13  6 RPM 74 27 11  3 RPM 67 26 10  10sec Gel (Pa) 28 14 9 PV (mPa * s) 33 10 11 YP (Pa) 70.5 14 5.5 LSRV (cP)100,000 100,000 80,000 Temperature 22.0 22.0 22.0 (° C.)

Example II

Background: Nr Wetaskiwin, Alberta, Drilled 222 mm hole to IntermediateCasing Depth of 1425 mMD and set casing at ˜86.2 degrees inclination inthe Rex Coal formation. Set and cement 177.8 mm casing.

Drilling Fluid: 60 m3 of mud is premixed with the following formulation:30 kg/m3 of natural bentonite is pre-hydrated in fresh water for 16hours. 3 kg/m3 of PolyVis II (MMH) is added over 2 hours. pH is raisedto 12.0 with caustic via chemical barrel over pre-mix tank. Fluidbecomes viscous. 50 kg/m3 of Potassium Sulphate is added.

Drilling in Coal: Intermediate casing shoe and cement are drilled outwith a 156 mm bit using water and then water is displaced over to thepre-mixed system, described above. This well was drilled horizontally inthe Rex Coal formation using the pre-mixed system.

Fluid Properties prior to drilling coal:Premix: 60 m3 circulating system.Depth: 1425 m (87.2 degrees inclination)

Funnel Viscosity: 55 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 64300 reading: 61200 reading: 60100 reading: 566 reading: 363 reading: 23PV (mPa·s): 3

YP (Pa): 29 Gels (Pa): 11/11

Filtrate (Fluid Loss, mls/30 min): no control

MBT: 30 Kg/m3

Potassium ion (mg/L): 25,000Fluid properties after drilling to 1451 m in Rex Coal formation:Depth: 1451 m (88 degrees inclination)

Funnel Viscosity: 66 s/L

Mud density: 1060 kg/m3pH: 11.5600 reading: 62300 reading: 55200 reading: —100 reading: —6 reading: —3 reading: —PV (mPa·s): 7

YP (Pa): 24 Gels (Pa): 6/10

Filtrate (Fluid Loss, mls/30 min): 60

MBT: 24 Kg/m3

Potassium ion (mg/L): 22,000

It was determined that the fluid viscosity remained substantially stabledespite drilling pure coal.

Thereafter drilling continued to 1845 m in Rex Coal formation with theaddition of 15×22.7 kg sacks of non-ionic starch (Unitrol Starch) forfluid loss control into 80 m3 system:

Fluid properties at depth 1845 m (91.4 degrees inclination):

Funnel Viscosity: 59 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 64300 reading: 56200 reading: —100 reading: —6 reading: —3 reading: —PV (mPa·s): 8

YP (Pa): 24 Gels (Pa): 9/11

Filtrate (Fluid Loss, mls/30 min): 19

MBT: 22 Kg/m3

Potassium ion (mg/L): 20,400

The addition of starch doesn't affect the rheology substantially.

After drilling to 2050 m in the Rex Coal formation the fluid propertieswere as follows (89 m3 system):

Depth: 2050 m (87.8 degrees inclination)

Funnel Viscosity: 85 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 80300 reading: 70200 reading: 65100 reading: 606 reading: 473 reading: 44PV (mPa·s): 10

YP (Pa): 30 Gels (Pa): 17/18

Filtrate (Fluid Loss, mls/30 min): 15

MBT: 25 Kg/m3

Potassium ion (mg/L): 22,500

It was determined that a mixed metal viscosified—natural bentonite typerheology can be maintained when drilling through coal with the presentsystem.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are know or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

1. A method for drilling in a coal containing a well into a formation,the method comprising: providing a mixed metal-viscosified drillingfluid including bentonite, and a mixed metal viscosifier of inorganicmagnesium/aluminum oxides and/or hydroxides and 1% to 10% potassiumsulfate (w/v); and circulating the drilling fluid through the well. 2.The method of claim 1 wherein providing the mixed metal-viscosifieddrilling fluid includes providing an aqueous-based drilling fluidincluding 15 to 45 kg/m3 bentonite clay, the mixed metal viscosifier ata weight ratio of 1:8 to 1:12, viscosifier to bentonite, a base tomaintain the pH above about 10.0 and potassium sulfate.
 3. The method ofclaim 1 wherein providing the mixed metal-viscosified drilling fluidincludes providing an aqueous-based drilling fluid including about 30kg/m3 bentonite clay, the mixed metal viscosifier in a quantity of about1:10 mixed metal viscosifier to bentonite with a pH controlled togreater than pH 11 and 1 to 5% potassium sulfate.
 4. The method of claim1 wherein providing the mixed metal-viscosified drilling fluid includes:mixing bentonite clay in water to form a bentonite mixture; adding themixed metal viscosifier to the bentonite mixture; adjusting pH togreater than above about pH 10; adding the potassium sulfate.
 5. Themethod of claim 4 further comprising adding any of fluid loss controladditives and/or lost circulation materials.
 6. The method of claim 1wherein providing the mixed metal-viscosified drilling fluid includesobtaining a drilling fluid with a yield point greater than 10 Pa.
 7. Themethod of claim 1 further comprising continuing circulation withentrained coal fines in the drilling fluid.
 8. The method of claim 7wherein rheology of the drilling fluid is substantially maintained. 9.(canceled)
 10. The method of claim 1 wherein circulating the drillingfluid is maintained while a coal seam is open to the drilling fluid. 11.The method of claim 1 wherein circulating the drilling fluid isinitiated substantially at surface.
 12. (canceled)
 13. (canceled) 14.The method of claim 4 wherein the pH is adjusted using caustic soda,caustic potash, potassium carbonate or soda ash. 15.-25. (canceled) 26.A method for producing a drilling fluid for drilling a wellbore, themethod comprising: providing a mixed metal-viscosified drilling fluidincluding clay and a mixed metal viscosifier of inorganicmagnesium/aluminum oxides and/or hydroxides; monitoring rheology of themixed metal-viscosified drilling fluid; and, adding potassium sulfate inan amount of 1%-10% (w/v) to the mixed metal-viscosified drilling fluidto avoid adverse rheological changes.
 27. The method of claim 26 whereinproviding the mixed metal-viscosified drilling fluid includes mixing anaqueous-based drilling fluid including 15 to 45 kg/m3 bentonite, a mixedmetal viscosifier at a weight ratio of 1:8 to 1:12 viscosifier tobentonite and a base to maintain a pH above about 10.0.
 28. The methodof claim 26 wherein providing the mixed metal-viscosified drilling fluidincludes mixing the aqueous-based drilling fluid including about 25 to45 kg/m3 bentonite and a mixed metal viscosifier in a quantity of about1:10 of mixed metal viscosifier to bentonite with a pH controlled togreater than pH
 11. 29. The method of claim 26 wherein monitoringrheology includes monitoring for fluid thinning and adding potassiumsulfate to avoid adverse rheological changes includes avoiding fluidthinning.
 30. The method of claim 26 wherein providing the mixedmetal-viscosified drilling fluid includes: mixing the clay in the formof bentonite in water to form a bentonite mixture; adding the mixedmetal viscosifier to the bentonite mixture; and adjusting pH to greaterthan about pH
 10. 31. The method of claim 26 further comprising addingany of fluid loss control additives and/or lost circulation materials.32. The method of claim 26 wherein adding the mixed metal-viscosifieddrilling fluid includes obtaining a drilling fluid with a yield pointgreater than 10 Pa.
 33. The method of claim 26 wherein adverserheological changes includes viscosity thinning.
 34. The method of claim26 the potassium sulfate is added prior to drilling into a coal seam.35. The method of claim 26 the potassium sulfate is added prior toinitiating the drilling process.
 36. The method of claim 26 wherein theclay is bentonite.